In plate heat exchangers of the kind described above the heat transfer plates and the linings are thin and get into direct contact with the heat exchange fluids, the result of which is that these parts of the plate heat exchanger will rapidly adopt the temperature of the heat exchange fluids and, thereby, will undergo quick changes in length. The frame plates, on the other hand, are considerably thicker than the heat transfer plates and the linings, and the frame plates do not get into direct contact with the heat exchange fluids. Thus, the frame plates with their one, inner sides are in contact with respective outer heat transfer plates of said plate package and with their other, outer sides are in contact with ambient air. Thereby, the frame plates undergo a smaller and, above all, slower change in length than the heat transfer plates.
The different changes in length of the heat transfer plates and the frame plates can lead to strength problems in the lining, in the connection between the lining and the outer heat transfer plate and in the portions around the ports of the outer heat transfer plate. In order to reduce these strength problems the lining, as initially mentioned, is arranged radially movable relative to the frame plate. An arrangement of this kind is described in WO 95/31687 A.
The radial movability of the lining relative to the frame plate necessitates a gap between the lining and the frame plate. The result of this gap is that the outer heat transfer plate, in an area around its port, has no support surface to abut against. As the fluid pressure in the plate package is high the outer heat transfer plate is subjected to bending in said area, the risk of material failure, primarily due to fatigue, being great. When the lining is displaced from a centered position in the through hole of the frame plate, as a result of the different changes in length of the frame plate and the plate package, the portion of the outer heat transfer plate, that is subjected to bending, becomes particularly large. For example, if the width of the gap is doubled the bending is almost quadrupled.